1. Field of the Invention
This invention relates generally to automated guided vehicles (AGVs), autonomous mobile robots (AMRs), remotely controlled robots (RCRs), and the like, and more particularly, to a multiple-degree-of-freedom (MDOF) vehicle wherein wheel slippage is eliminated and/or rendered inconsequential by permitting the distance between two drive chassis on the same vehicle to be monitored and using a resulting distance measurement signal to control vehicle operation.
2. Description of the Related Art
The ability of AGVs to perform industrial functions has increased as computer technology advances. Oftentimes, these vehicles are guided by wires which are embedded into the floor. AMRs, on the other hand, navigate autonomously, and are useful in situations where in-floor guide-wiring is not available. In hazardous environments, such as in nuclear power plants and radioactive storage sites, RCRs are particularly useful. All of these vehicles have drawbacks which prevent them from achieving optimum levels of operation.
Conventional vehicles use either a differential drive design which employs two drive wheels, each with its own motor, or a tricycle design, where one wheel is steered and driven. Vehicles which employ these operating arrangements are easy to control and more maneuverable than automobiles, for example. However, in many applications, floor space is quite limited, and there is therefore a need for even greater maneuverability. This is particularly true in environments which have not been designed to accommodate automatic vehicles.
One known system which improves maneuverability is the so called "synchro-drive" arrangement. Vehicles which employ this known drive system typically have three driven and steered wheels that are mechanically linked to one drive motor and one steering motor. These vehicles provide only two degrees of freedom. However, the three wheels can be steered into any direction, but are always parallel to each other. Thus, although this design permits the vehicle to move in all directions, there is not control over the orientation of the vehicle body, since only the wheels turn.
Full control over travel direction and orientation has been achieved with special wheels which can roll sideways. Usually, such vehicles are driven by three or four independent drive motors. Although these vehicles are useful in some applications, their utility is limited to usage on smooth and regular surfaces. Most industrial applications do not provide such smooth surfaces.
For purposes of the present discussion, MDOF vehicles are limited to have full-sized, conventional wheels. Such vehicles are excellent for transport tasks in confined spaces. In theory, MDOF vehicles are extremely maneuverable, they are capable of turning in a confined space; they can move sideways, and can perform maneuvers which allow the vehicle to move along a mathematically optimal trajectory. Good MDOF designs are capable of reducing significantly the amount of floor space necessary to achieve safe operation of the vehicle.
Although a vehicle which employs more than two independently controlled axles offers exceptional advantages from the standpoint of maneuverability, exceptional difficulties are encountered from the standpoint of control. FIG. 1 is a schematic representation which shows a typical design of a MDOF vehicle 10, specifically a four-degree-of-freedom vehicle, which is known to persons of skill in the art. As shown, vehicle 10 has a left drive wheel 11 and a right drive wheel 12 which, in addition to being rotated to perform the drive function, they are also steerable, as shown by the circular arrows. Four castors 13, arranged at the vehicle corners, provide stability. In this known arrangement, driver wheels 11 and 12 are set apart by a fixed distance d. A known vehicle employing the drive system of FIG. 1 having two tricycle drives and employing a total of four motors was developed and built at the Oak Ridge National Laboratory, and denominated as HERMIES-III.
Experiments have established that this known design yields large position errors resulting from severe wheel slippage after performing certain maneuvers. Similar results, plus severe oscillations with servo errors in the drive and steering motors have been reported in six-degree-of-freedom vehicles employing conventional designs, such as the PLUTO machine developed at Carnegie-Mellon University.
FIG. 2 is useful to illustrate the concept of instantaneous center of rotation ("ICR") as described in U.S. Pat. No. 4,932,489. As shown in this figure, a vehicle 20 is attempting to maneuver around the end of a wall 21. This concept for controlling the trajectory of a MDOF vehicle assumes that a higher-level trajectory planner has determined that points A and B on the vehicle should momentarily travel in the directions .alpha., illustrated by arrow 22, and .beta., illustrated by arrow 23, as shown in FIG. 2. The desired trajectory, which is designated as 25 in the figure, can be prescribed by a guide wire in AGV applications, or an obstacle avoidance system in AMR applications.
The ICR concept is borrowed from the areas of machine design and kinematics. It utilizes the fiction of an imaginary point, illustratively point 26 in the figure, about which a rigid body (e.g., vehicle 20) appears to be rotating momentarily, i.e., for an instant dt, while the body is rotating and translating. In pure translatory motion, the ICR is located at a distance .infin. from the body. A special case of translatory motion exists when both wheels are parallel to the longitudinal axis of motion. This configuration corresponds to the widely used differential drive where two wheels are located on the same axis, but are driven by individual motors. Hereinafter, this shall be termed the "normal" configuration, and, by contrast, the term "crabbing" is used when at least one wheel is not oriented parallel to the longitudinal axis of the vehicle.
In FIG. 2, ICR 26 is situated at the cross point of the two normals to the steering directions .alpha. and .beta.. Then, the orientation of the two wheels is set normal to the two position vectors r.sub.1 and r.sub.2. Clearly, this orientation of the two wheels will cause rotation around the ICR and consequently, rotation around the ICR results in points A and B momentarily having the required steering directions. However, the velocities of the wheels must maintain the relationship: ##EQU1## Eq. 1 is known in the literature as the "Rigid Body Constraint." It should be noted that V.sub.1 will be independent from V.sub.2 when r.sub.1 +r.sub.2 =.infin., i.e., in the normal configuration. It is also important to point out that the ICR concept can be applied to vehicles with any number of degrees of freedom, e.g., 4 drive/4 steer kinematics.
One theoretically interesting type of crabbing occurs when both wheels are turned 90.degree. sideways, allowing the vehicle to travel sideways in confined areas. However, this important capability is not necessarily feasible with existing vehicles, as will be shown hereinbelow.
The problem with the ICR concept is that Eq. 1 must be met accurately, i.e., the ratio between the two velocities must be maintained, or otherwise wheel slippage will occur. It is a problem with known arrangements that conventional DC motor velocity control loops do not precisely follow the prescribed velocity profile during transients. The integral of these errors over time translates into permanent (not transient) position errors for each wheel. In other words, even if both control loops had the same velocity reference commands, each loop would still end up with different pulse counts from the encoders. For the simple case of the normal configuration, the pulse count difference As causes an orientation error .DELTA..theta. that can be expressed as: EQU .DELTA..theta.=.DELTA.s/b Eq. 2
If not corrected immediately, .DELTA..theta. may cause a very large lateral position error during subsequent motion. Even under steady-state conditions, mechanical disturbances on the wheels, e.g., bearing friction, will cause the wheels to rotate temporarily at different speeds, and therefore generate different numbers of pulses.
The errors discussed hereinabove with respect to the 4-DOF mobile robot are not the only significant ones. These vehicles are subject to additional errors beyond the orientation error .DELTA..theta.. The errors are generally so grave that crabbing motion is generally unfeasible with the design of FIG. 1. As discussed hereinabove, any difference in pulse counts as generates a rotary motion about the center of the robot. Even in normal configuration, this rotary motion will cause an orientation error .DELTA..theta.. In the normal configuration this rotary motion is considered "permissible," since it does not cause slippage. Slippage causes much more severe problems than do position errors caused by inadequate control. This results from the fact that slippage errors cannot be corrected, while control-type errors can. By contrast, when operating in crabbing motion, the encoder pulse difference .DELTA.s does cause slippage.
FIG. 3 is useful to illustrate the effect of .DELTA.s during crabbing. Assume that right wheel 31 is stationary while left wheel 32 rotates through the extra distance .DELTA.s. Decomposing .DELTA.s into its orthogonal components, .DELTA.s.sub.x =.DELTA.s sin.alpha. and .DELTA.s.sub.y =.DELTA.s cos.alpha., it is clear from FIG. 3 that .DELTA.s.sub.y is completely lost to slippage because the distance d between the drive wheels of this known arrangement is physically fixed.
It has been concluded by the inventor herein that there is a need for a mechanical arrangement which accommodates temporary discrepancies between pulse counts, until the controllers catch up to correct the problem. The inability of known arrangements to accommodate position errors .DELTA.s is the reason these arrangements shake and rattle during operation. These known arrangements attempt to accommodate the position errors through unintentional compliance, such as backlash, resulting in extensive wheel slippage.
In addition to the inherent control problems encountered when endeavoring to maintain accurate velocity ratios between the drive wheels during crabbing, other problems exist in most known systems. One such problem results from unequal wheel diameter.
In order to improve traction, most vehicles use rubber wheels. It is, however, difficult to manufacture rubber wheels with exactly the same diameter. In addition, unequally distributed loads will compress one wheel slightly more than the other, resulting in a difference between the effective rolling radii of the wheels. As the motor controller attempts to translate wheel revolutions into linear travel distance, the computation is based on nominal wheel diameter, and therefore, even a small difference from the nominal diameter renders the computation inaccurate. The result is a relative position error .DELTA.s. It is noteworthy that this source error is constant, at least until the loading of the vehicle is changed, resulting in a constant wheel slippage rate in the conventional MDOF vehicle.
Additional error is introduced by misalignment of the wheels. This will result in slippage in any vehicle. However, in certain MDOF vehicles, the problem is exacerbated by the fact that backlash and limitations in the resolution of the steering mechanism introduce much greater misalignment errors than mechanically fixed wheels do. Such misalignment will render Eq. 1 inaccurate and increase the wheel slippage.
It is, therefore, an object of this invention to provide a multiple-degree-of-freedom vehicle which can provide excellent maneuverability, while avoiding wheel slippage.
It is another object of this invention to provide a multiple-degree-of-freedom vehicle which can provide sideways (crabbing) motion and the ability to negotiate tight turns.
It is also an object of this invention to provide a multiple-degree-of-freedom vehicle which can provide a high payload capacity.
It is a further object of this invention to provide a multiple-degree-of-freedom vehicle which can provide improved traction with all wheels driven.
It is additionally an object of this invention to provide a multiple-degree-of-freedom vehicle which can provide high static and dynamic stability.
It is yet a further object of this invention to provide a multiple-degree-of-freedom vehicle which can execute maneuvers without the error which results from wheel slippage.
It is also another object of this invention to provide a multiple-degree-of-freedom vehicle which can execute maneuvers without the oscillations and other errors which have plagued prior art designs.
It is yet an additional object of this invention to provide a multiple-degree-of-freedom vehicle which can execute maneuvers quietly, without the grinding and shaking noises of prior art designs.
It is still another object of this invention to provide a multiple-degree-of-freedom vehicle which overcomes problems resulting from differences in effective wheel size caused by wear, manufacturing tolerances, or loading.
It is a yet further object of this invention to provide a multiple-degree-of-freedom vehicle which overcomes problems resulting from wheel misalignment.
It is also a further object of this invention to provide a robotic implementation of a multiple-degree-of-freedom vehicle which can be operated intuitively by a human operator using a remote joy-stick.
It is additionally another object of this invention to provide a multiple-degree-of-freedom vehicle which overcomes problems resulting from non-point contact of a wheel with the floor.
A still further object of this invention is to provide a multiple-degree-of-freedom vehicle which overcomes problems resulting from operating a vehicle on an uneven floor.
Yet another object of this invention is to provide a multiple-degree-of-freedom vehicle which can function adequately even when one or more motors on an axle fail.
Another object of this invention is to provide a multiple-degree-of-freedom vehicle which enjoys the benefits of a widened wheelbase without sacrificing maneuverability.